for some reason, the rig I got online had one of Robotnik's arms lodged further in his body than the other. I fixed that, and I made the arms have some kind of depth... It should be fine now. May this also be used in the near future?

So, why do polygons that are right next to the camera have a distortion where it doesn't stay on the same plane? I'm guessing it's because the 4 points cannot go beyond coordinates in this overscan the Saturn has around the screen.

So, can you make the shadows normal sprites to get all of the benefits of transparent pixels? Just get rid of perspective distortion and instead use skewing, because it's a minor detail. Would that be a good trade-off?

Actually, you might not even need skewing or even rotation for circular shadows, because it looks the same from all angles. So you might only need to make it a scaled sprite, which can let it be transparent.